1. Field of the Invention
The present invention relates to a method of controlling hand-over (change of radio communication base stations to be connected) performed by a radio communication terminal such as a portable telephone or the like.
2. Description of Related Art
FIG. 1 is a schematic diagram illustrating a radio communication system to explain hand-over performed by radio communication terminal. In FIG. 1, cell A indicates an area in which radio communication terminal 31 can perform a radio communication with radio communication base station 32, and cell B indicates an area in which radio communication terminal 31 can perform a radio communication with radio communication base station 33. Hand-over performed by radio communication terminal 31 refers to an operation in which radio communication terminal 31 which is communicating with radio communication base station 32 change radio communication with radio communication base station 32 to radio communication with radio communication base station 33 when radio communication terminal 31 moves from cell A to cell B, When performing hand-over from radio communication base station 32 to radio communication base station 33, radio communication terminal 31 is required to change the frequency used for radio communication from a frequency available in radio communication base station 32 to a frequency available in radio communication base station 33.
Such inconvenience can be solved by using code division multiple access (CDMA) for radio communication. In a CDMA system, radio communication terminal 31 can distinguish between a signal transmitted from radio communication base station 32 and a signal transmitted from radio communication base station 33 and can simultaneously receive both signals. Therefore, with the hand-over performed by radio communication terminal 31 using CDMA, when radio communication terminal 31 moves from cell A to cell B while communicating with radio communication base station 32, radio communication terminal 31 adds communication with radio communication base station 33 while maintaining the communication with radio communication base station 32, radio communication terminal 31 being set to have simultaneous reception from both radio communication base stations 32 and 33. Thereafter, when the received power of signals transmitted from radio communication base station 32 becomes low, radio communication terminal 31 finishes the communication with radio communication base station 32 and communicates only with radio communication base station 33.
A radio communication terminal using such CDMA includes a plurality of receivers called a finger receiver and hence has the capability to simultaneously receive signals transmitted from three radio communication base stations. This feature of simultaneous reception allows hand-over in a radio communication terminal using CDMA to be classified into three general cases: (i) When communicating with one or two radio communication base stations, the radio communication terminal adds communication with another radio communication base station; (ii) When communicating with two or three radio communication base stations, the radio communication terminal finishes the communication with these radio communication base stations; (iii) When communicating with three radio communication base stations, the radio communication terminal changes communication with one of the radio communication base stations to communication with another radio communication base station that is not currently communicating with the radio communication terminal.
The processing in case (iii), which has the most problems of the three cases, will be described in detail with reference to FIG. 2.
FIG. 2 shows that when radio communication terminal 41 is communicating with radio communication base stations 42, 43 and 44, radio communication base station 45 becomes a candidate for hand-over with which radio communication terminal 41 starts communication. FIG. 3 shows a change in the received power of radio communication terminal 41 over time. FIG. 4 shows a sequence of messages exchanged between radio communication terminal 41 and radio communication base stations 42 to 45.
As shown in FIG. 2, radio communication terminal 41 is communicating with radio communication base stations 42, 43 and 44. Radio communication terminal 41 measures the received power of a signal transmitted from radio communication base station 45 during communication, As shown in FIG. 3, the received power of a signal transmitted from radio communication base station 42 is lowered, whereas the received power of a signal transmitted from radio communication base station 45 is increased. As shown in FIG. 4, immediately before the received power of a signal from radio communication base station 45 exceeds the received power of a signal from radio communication base station 42, radio communication terminal 41 transmits message 61 to radio communication base stations 42, 43 and 44 to notify the stations that the result of measurement of the received power of a signal transmitted from radio communication base station 45 satisfies the conditions for hand-over.
Upon receipt of message 61, radio communication base station 42 finishes communication with radio communication terminal 41, and radio communication base station 45 starts communication with radio communication terminal 41. Each of radio communication base stations 43 and 44 transmits message 62 to radio communication terminal 41 to notify the terminal of conditions to communicate with radio communication base station 45 that is a target for hand-over, such as timing and the like.
Upon receipt of message 62, radio communication terminal 41 sets the notified timing information and the like to the finger receivers thereof, and transmits message 63 to radio communication base stations 43, 44 and 45 to notify the base stations that procedure to set the notified communication conditions has been finished.
Subsequently, in preparation for the next hand-over, each of radio communication base stations 43, 44 and 45 transmits message 64 to radio communication terminal 41 to cause terminal 41 to renew hand-over information such as information about the periphery cells of terminal 41 and the like, thereby finishing the hand-over processing. Radio communication terminal 41 then examines the qualities of signals received from radio communication base stations in the peripheral cells in accordance with received massage 64 to renew hand-over information.
In radio communications, fading may occur due to buildings and geographical features, causing fluctuation of the received power from each radio communication base station. This fading exerts a large influence on the processing for the case (iii). FIG. 5 explains the influence exerted on the processing for the case (iii).
As shown in FIG. 5, qualities of signals received from radio communication base stations may change greatly over time as in cell C or cell D. Now consider hand-over which is performed under radio signal propagation circumstances as shown in FIG. 5. Initially, radio communication terminal 41 communicates with the radio communication base stations in cells A, B and C. Each time the relationship in the qualities of received signals between cells C and D alternates, radio communication terminal 41 performs hand-over to switch between the radio communication base stations in cells C and D. Thereafter, when the qualities of received signals from the radio communication base stations in cells A and B deteriorate and the qualities of received signals from the radio communication base station in cell E is increased, the radio communication base station in cell E becomes a candidate for hand-over. However, because radio communication terminal 41 performs hand-over to switch between the radio communication base stations in cells C and D, it cannot perform hand-over to the radio communication base station in cell E. This causes a problem that a call is undesirably disconnected due to the deterioration of the qualities of received signals from the radio communication base stations in cells A and B.
The reason that hand-over to the radio communication base station in cell E can not be performed is that since the sequence of hand-over is being performed between radio communication terminal 41 and the radio communication base stations in order to perform hand-over to switch between the radio communication base stations in cells C and D, as illustrated in FIG. 5. message 61 which notifies that cell E has satisfied the hand-over conditions and which is transmitted from radio communication terminal 41 to the radio communication base stations, is disregarded by the radio communication base stations. In this way, in situations where hand-over to switch between two cells C, D is frequently performed, the hand-over that is necessary to maintain communications with radio communication terminal 41 cannot be performed due to unnecessary hand-over that has been frequently performed.
Several techniques have been proposed to solve this problem. For example, JP 2001-16932A (hereinafter referred to as Patent document 1) discloses a technique to detect such unnecessary hand-over through a learning means. JP2004-40338A (hereinafter referred to as Patent document 2) discloses a technique wherein the number of hand-over operations between two radio communication base stations is counted, and hand-over between the radio communication base stations is prohibited for a predetermined period of time when the counted value exceeds a predetermined value.
However, with the invention of Patent document 1, it is time-consuming to build the learning means, and unnecessary hand-over caused by fading cannot be prevented in a simple manner. With the invention of Patent document 2, the repetition of unnecessary hand-over a predetermined number of times cannot be avoided.